Great advances in the field of wireless communications have been made over the past twenty years, and continue to be made. These advances both improve the quality of communication, e.g., the clarity and reliability of communication, and improve the geographic coverage of such wireless communications. As industry strives to provide a wireless communications capability that covers the entire globe, factors such as economic viability dictate that space-based transceivers be employed as opposed to ground infrastructure. However, systems that employ ground infrastructures remain technologically advantageous and economically preferable in identified population centers where a great deal of bandwidth is required in a relatively small area. Thus, two types of wireless communication, i.e., ground infrastructure cellular, and space-based satellite systems have emerged.
Modems are well-known in the art of digital communications. The word modem is a contraction of the words modulator and demodulator. A modem is typically used to send digital data over in transmission line. The sending modem modulates the data into a signal that is compatible with the transmission line and the receiving modem de-modulates the signal back into digital data. Wireless modems are frequently used in cellular and satellite communication systems for converting data into radio signals and visa-versa.
Modems utilize any number of a different modulation schemes that transmit digital data. For example, modems used in transmitting digital data over telephone lines would use frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be inserted into the relatively small bandwidth available on normal voice grade phone lines. In satellite communication systems, however, much higher speeds of communication are available through the use of these and other sophisticated modulation techniques.
As discussed above, because of increased use of the Internet, the use of modems has risen substantially. In the early stages of Internet use, the Internet was primarily utilized through conventional landlines, i.e., plain old telephone service (POTS) lines. However, as use of the Internet became more prolific, and users demanded greater accessibility, means for wireless Internet service became established and is now fairly common. However, in many places of the world, where cellular wireless communication systems are not in place to provide telephone or Internet access, satellite communication systems can be used.
FIG. 1 is a block diagram of a satellite telecommunication system. In FIG. 1 satellite 2, located in space 4, transmits via spot beams 8A–8C, data and voice information. Each spot beam 8 comprises not more than 75 communication channels, suitable for packet data transfer, numbered 01 to 75. Each channel can be either 125.0 kHz or 156.25 kHz, as discussed in greater detail below.
The Internet operates in a packet switched environment, as opposed to the circuit switched environment of normal landline (and even most wireless) telephone systems. “Circuit Switched” is the most familiar technique used to build a communications network. It is used for ordinary telephone calls. Circuit switching means that each user has sole access to a circuit (functioning equivalent to a pair of copper wires) during network use. Essentially, it is a direct link, through the paired copper wires, and a network between two end points. “Packet Switching” however is somewhat different then circuit switching. Packet Switching is similar to message switching using short messages. Any message exceeding a network defined maximum length is broken into short units, known as “packets” for transmission. The packets, each with an associated header, are then transmitted individually through the network. In a packet system a number of users may simultaneously receive the transmitted data. Each packet is individually addressed, which is how they get to the correct recipient. In essence, there is no completed direct connection between the sending and receiving units. It is possible, and quite frequently it does happen, that an outgoing link may not be available in which case a packet is placed in a queue until the link become free to use. A packet network is formed by links which connect packet network equipment.
Wireless packet data systems are being developed based on the General Packet Radio Service (GPRS) standard. This has been enhanced for satellite operation, the enhancement being named Geo-Mobile Packet Radio Service (GMPRS) standard. GMPRS is an advanced data transmission mode that does not require a continuous connection to the Internet, as with a standard home modem. Instead, a system that employs GMPRS uses the network only when there is data to be sent, which is more efficient in wireless communication systems, where power and spectrum are scarce resources. The base frequency bandwidth for packet data transfer is either 156.25 kilohertz or 125 kilohertz depending on the configuration used. In addition to packet data transfer (a channel allocated for packet data transfer is referred to as a packet data channel, or PDCH), the same satellite channels are used for packet random access channel (PRACH) requests. A PRACH request is used by a remote terminal to request an allocation of resources (i.e., the channel) in order to transfer data from the terminal. Since the payload requirements for a PRACH request is significantly lower than the packet data transfer, the PRACH occupies only 31.25 kHz. In order to make efficient use of the bandwidth, the modified air interface allows multiply PRACH's to occupy the same space as a single PDCH (in either the 156.25 kilohertz channel, or the 125 kilohertz channel). This is explained with reference to FIG. 2.
FIG. 2 illustrates a first wideband channel for transferring packet data and random access channel requests in a satellite communication system utilizing the GMPRS standard. As seen in FIG. 2, there is an odd frame 201 and an even frame 202. In odd frame 201 and even frame 202, there are eight time slots of 5 milliseconds a piece. Therefore, each channel frame transmission time is 40 milliseconds. The time slots are denoted time slot A (TS-A), time slot B (TS-B), etc. In FIG. 2, the transmission bandwidth is 125.00 kHz. In FIG. 2, TS-A and TS-C through TS-H of odd frame 201 are dedicated to packet data channel (PDCH) transfers. TS-B, however, is dedicated for a PRACH. There are four PRACH transmissions (PR-1, PR-2, PR-3 and PR-4), each occupying 31.25 kilohertz of bandwidth. In even frame 202, however, there is no PRACH transmission. PRACH transmissions occur in an odd frame or an even frame, but not both, in any given channel.
FIG. 3 illustrates a second wideband channel for transferring packet data and random access channel requests in a satellite communication system utilizing the GMPRS standard. Although the transmission bandwidth is 156.25 kHz, it can been seen in odd frame 301, the top frequency band of 31.25 kilohertz is crossed-hatched to represent that no data transmission occurs in that frequency band. Thus, all transmissions occur around the frequency carrier signal from 0–125 kilohertz. In even frame 302, the first 31.25 kHz is cross hatched to represent that no data transmission is occurring in that frequency band. In FIG. 3, TS-C of even frame 302 contains the PRAQCH transmission.
Normally, satellite radio modems (SatMods) are assigned to channels on a fixed basis. SatMod assignment is performed by a device call the Packet Resource Management System (PRMS). When a new channel (or sub-band) is assigned, the PRMS looks through a list of available PRACH SatMods, and assigns the according to which slots are free. As has been discussed, the wideband packet data channels (PDCH) are periodically overlaid with multiple narrowband channels. However, if conventional techniques were used to receive a PRACH transmission as with a PDCH transmission, it would be necessary to provide up to five times the modem hardware then is needed for data transfer alone, at great expense.
FIG. 4 illustrates a first example of an inefficient allocation of satellite radio modems for transferring PDCH and PRACH transmissions utilizing the GMPRS standard. In FIG. 4, SatMod 1 has been assigned to channel 50. It will receive all the packet data transfers (PDCH). However, SatMods 2–5 have been assigned to receive one PRACH transmission each. SatMod 1 can be used for channel 50 odd frame, as well (but not at the same time), but no other channel. SatMods 2–5 can likewise be used only for channel 50 odd and even frames. This is the same for all 75 channels. Therefore, in 75 channels, 75 PDCH SatMods are needed, and 300 PRACH SatMods (75 channels×4 PRACH SatMods/channel=300 PRACH SatMods), for a total of 375 SatMods. The significant component of a SatMod is a high-speed digital signal processor (DSP) that is contained in the SatMod. A SatMod will be considered to be one DSP.
FIG. 5 illustrates a second example of an inefficient allocation of satellite radio modems for transferring packet data and random access channel requests utilizing the GMPRS standard. The allocation of SatMods in FIG. 5 is only slightly more efficient than in FIG. 4. Here, PDCH SatMod 1 is used for PRACH (PR) SatMod for PR-4 (in TS-B), while SatMods 2–4 are used in PR-1, PR-2 and PR-3, respectively. In this example, therefore, for 75 channels, 75 PDCH SatMods are needed, and 225 PRACH SatMods are used (75 channels×3 PRACH SatMods/per channel=225 PRACH SatMods) for a total of 300 SatMods.
FIG. 6 illustrates of a third example of an inefficient allocation of satellite radio modems for transferring packet data and random access channel requests utilizing the GMPRS standard. FIG. 6 represents an inefficient allocation of both SatMods and bandwidth resources. In FIG. 6, SatMod 1 will be used for PDCH transfers in channel 50, even and odd frames. In this case, though, only one PRACH is transmitted, PR-4, alleviating the need for any additional SatMods, because for each channel, the one SatMod for the channel can easily handle the smaller bandwidth PRACH, as well as the much higher bandwidth PDCH transfers. At a minimum, therefore, only 75 SatMods need be used; but, instead of completely utilizing the bandwidth capacity of a time slot in each channel's even and odd frame, only 31.25 kHz (PR-4) of the PRACH timeslot is being used. Alternatively, a single PRACH SatMod can handle the PRACH (in this example PR-4) for each channel, thereby doubling the number of SatMods to 150. In both the 125.0 kHz and 156.25 kHz wideband channel, 75% of the PRACH channel's time slot capacity is wasted.
Thus, there is a need for efficiently using modems in a satellite communication system that transfers packet data and random access channel requests utilizing the GMPRS standard, and a need for efficient utilization of channel bandwidth in such a system.